Hmm, you're right, when inserting a bit ly link into a post, the forum seems to modify it to go via an ad. I'm not sure I'd describe what I saw as malicious, but it's certainly unwelcome.

I guess nothing in this life is for free. I suspect if this was a paid forum then the problem would go away - and that would be $65 a year. I appreciate it's annoying but with a quick test it looks like other, non-bit ly links are unaffected. Perhaps another URL shortening service would also be OK, or instead perhaps you could direct link instead of going via a redirection service.

I am able to hide links from non-members. That may also adjust the way that it works, but would be less useful for forum users.

If the ads become too intrusive, inconvenient or inappropriate, I may have to take down the forum altogether.

Why do you need to use a URL shortener at all? Seeing as the target of the links seems to be Conforums, which is about to go offline, the links will all need to be changed anyway. Why not link directly to the target page?